1. Field of the Invention
This invention relates to a wafer alignment method and apparatus and particularly to a stepper alignment method and apparatus for wafer exposure system.
2. Description of the Prior Art
In semiconductor manufacturing process, one of the steps is to form a circuit layout on the wafer surface. FIG. 1 illustrates a wafer exposure scheme which uses a stepper (not shown in the figure) to project and transfer a circuit layout on a mask 1 to a shot site 21 of the wafer 2.
The accuracy of the exposure has great impact to the yield of the wafer. In order to make the stepper project the layout pattern on the mask 1 to the wafer at a precise location desired, a robot arm is used to make a pre-alignment for the wafer 2 before exposure operation is performed. Then the wafer 2 is placed on a chuck of the stepper for exposure operation. Some degree of error is usually unavoidable in the movement of the robot arm and thus will result in wafer off-set. To remedy this problem, a plural number of first alignment marks 22 are made just beyond the lateral sides of the shot site of the wafer. The mask 1 also has second alignment marks 12 to mate against the first alignment marks 22 for precisely projecting the circuit layout from the mask 1 to the wafer 2 for exposure process.
Using conventional art to perform the exposure process set forth above, the wafer 2 must be pre-aligned by a robot arm. Then the second alignment marks 12 of the mask 1 should be mated against the first alignment marks 22 on the shot site correctly before exposing process start. This procedure is time-consuming and has negative impact on wafer 2 production efficiency. Furthermore the first alignment marks 22 tends to fade away because of the Chemical-Mechanical Polishing (CMP) process. This will also impair the yield of the wafer 2. All this begs for improvement.